Electric gaseous discharge device



Nov. 8, 1938. w. ELENBAAS ET AL 2,

ELECTRIC GASEOUS DISCHARGE DEVICE Filed Aug. 16, 1957 INVENTOR WiHem' Elenbaas Aart van Wgk BY H A ORNEY Patented Nov. 8, 1938 UNITED STATES ELECTRIC GASEOUS DISCHARGE DEVICE Willem Elenbaas and Aart van Wyk, Eindhoven,

Netherlands, assignors to General Electric Company, a corporation of New York Application August 16, 1937, Serial No; 159,430

In Germany August 31, 1936 (I 2 Claims.

The present invention relates to electric gaseous discharge devices and particularly to devices having thermionic electrodes.

A particular object of the invention is to pro- 5 vide a novel gaseous discharge device having a long useful life. Another object of the invention is to provide means to shift the discharge from one electrode to another within a discharge device after a desired interval. Still other objects and advantages of the invention will appear from the following detailed specification or from an inspection of the accompanying drawing.

The invention consists in the new and novel combination of elements hereinafter set forth and claimed.

One of the limiting factors in the life of the modern electric vapor discharge lamps with activated cathodes is the life of the activating coating. This coating, which is vital to the star ing of thedischarge at a low potential, is gradually volatilized and sputtered off during operation, and eventually leads to deactivation of the electrodes, and at the same time causes undesired tube blackening. This has led to a proposal to utilize these activated electrodes only at starting, and causing the discharge thereafter to run between uncoated electrodes of tungsten or the like which are operated at a temperature sufiicient to provide the necessary electron emission.

In accordance with this proposal the necessary transfer between the electrodes was accomplished by connecting the starting and running electrodes in parallel, with the arc path to the uncoated running electrode shorter than that to the coated electrode. As the vapor pressure increases after starting the voltage gradient increases and the running electrodes. are also heated at the same time by the discharge, increasing their thermionic emission until eventually the discharge transfers to these running electrodes due to the shorter arc path, the moment of transfer being determined by the spacing between the starting and running electrodes. This structure has proved to have many desirable features, and has given successful results.

We have now found, however, that the operation of these lamps is distinctly improved by a novel arrangement of our invention wherein the starting electrodes are connected to the running electrodes (or to the energy source) through a resistance which is not traversed by the are current to the running electrodes. With this novel arrangement the distance between the starting and running electrodes is materially reduced whereby the possibility of vapor condensation on I ing in an arc restricting baffle which directs the discharge onto the running electrode and thus' facilitates the heating thereof to the desired high temperature. This bafile furthermore confines any of the metal spattered off the electrodes, 15 and prevents blackening of the arc tube.

In alternating current operation, the tube is provided with two sets of starting and running electrodes; with direct current one such set of electrodes is sufficient which is used as cathode, 20 while the anode is constructed in a conventional manner, where desired.

During ignition of the tube, the discharge first attaches itself to the activated starting electrode (which is heated by a special heater current, or 25 more expediently by the discharge), so that the current flows over the resistance. The tube heats up, whereby the vapor pressure and the voltage gradient (voltage drop per unit of length of the discharge path) rise. As soon as the cur- 30 rent can flow over the main running electrode with a smaller voltage loss it transfers itself thereto. The time interval between the ignition and this transfer is regulated in our novel structure by adjusting the size of the resistance, and 35 is preferably adjusted in such a manner that the auxiliary electrode conducts the current in the low pressure operation and the main electrode in the high pressure operation of the discharge. 7 Our novel structure has the further 40 advantage that this adjustment of the resistance can be undertaken after the lamp has been sealed off, so that the above mentioned time interval can be changed after finishing the tube, without the need of changing anything inside of the tube. The distance between the main electrode and the auxiliary electrode is so chosen, independent of the above mentioned time interval between the ignition and the passing over of the discharge, 50 to be most favorable in regard to the ignition voltage and the heating of the tube.

For the purpose of illustrating ourinvention we have shown a preferred embodiment thereof in the accompanying drawing, in which- 55 Fig. 1 shows a longitudinal section of this tube, while Fig. 2 shows a main running electrode of this tube in perspective.

As shown in the drawing, the discharge tube I, consisting of quartz and suited for emitting visible light or ultra-violet rays, is cylindrical and contains at each end a helical starting electrode 2 arranged along the axis of the tube. This starting electrode is covered with an activated coating, such as a mixture of barium oxide and strontium oxide. Before each electrode 2 there is a main running electrode 3, which consists of a tungsten wire that has an approximately circular part 4 which lies in a plane perpendicular to the tube axis (see Fig.2) and a projecting part 5 lying in the tube axis. The tube I preferably further has two quartz screens or bailies 6, each of which has a central opening I, each bafile being located a short distance from the projecting electrode part 5.' To facilitate equalization of the gas pressure and vapor pressure in the various parts of the tube 1 the screens 6 are further provided, where desired, with one or more openings at their rims.

Between the inlead wire 8 of each electrode 3 and the inlead wire 9 of each starting electrode 2, a resistance I0 is connected, preferably outside of the discharge space. This resistance is conveniently arranged in the conventional base (not shown) so that the finished discharge tube has only two contacts which, as shown in the drawing, are attached to an alternating current source 12 through an external choke coil 1 l.

The tube is provided with a rare gas filling, such as argon at a pressure of the order of 5 mm. mercury, and moreover contains a quantity of mercury, which preferably is so limited that the entire amount is already vaporized before the tube has reached its normal operating temperature, so that the mercury vapor is unsaturated during operation and the vapor pressure changes only a little during fluctuations of the supply voltage or during changes in the cooling conditions.

When put into operation, the current flows first between the starting electrodes 2. The discharge which proceeds from these auxiliary electrodes is constricted by the screens 6 and is conducted along the electrode parts 5 which are heated up by the discharge. When the vapor pressure (and therefore the voltage gradient) has reached a sufficiently high value and the electrode parts 5 have been brought to a sufficiently high temperature, the discharge passes over from the starting electrodes 2 onto the main running electrodes 3, where it is largely confined to the ends of the electrode parts 5, whereby the position of the discharge is stabilized. In normal operation, the starting electrodes 2 carry no current or only a negligibly small current. The time interval between the ignition and the transfer of the discharge depends on the size of the resistances l0, and is thus readily controlled.

In a certain tube the distance between the electrode parts 5 was about 18 cm. and the distance between the auxiliary electrodes was about 19 cm., while the resistances II) had a value of about 5 ohms and the current strength of the tube amounted to 4.5 amp. after reaching the normal operating condition. The burning voltage of the discharge shortly after ignition was about 20 volts, whereas the burning voltage in the normal operating condition was volts, while it amounted to about 0 volts at the moment when the discharge passed over from the auxiliary electrodes onto the main-electrodes.

In the tube illustrated in the drawing the starting electrodes are shown as being of a type heated by the discharge, but it is to be understood that these electrodes may also be heated where desired by means of a special heater current which is switched off after the discharge has transferred to the main runningelectrodes. The latter construction however is not as simple as the one shown.

It is also to be understood that the invention is not limited to the particular device illustrated, but that various changes, substitutions and omissions, within the scope of the appended claims, may be made therein, without departing from the spirit of the invention.

Whatwe claim as new and desire to secure by Letters Patent of the United States, is:

1. An electric gaseous discharge device comprising a sealed envelope containing a gaseous atmosphere consisting in part of vaporizable material, at least three electrodes within said envelope, one of said electrodes being at one end of said envelope and the other two near the opposite end of said envelope but at different distances from the first electrode,'the electrode which is nearer said first mentioned electrode being an unactivated electrode of a refractory metal capable of thermionic emission when heated while the more remote of said electrodes is an activated thermionic-cathode, and a resistance connected between the two latter electrodes and traversed only by the current to the activated cathode which facilitates the transfer of a discharge from the activated starting electrode to the unactivated running electrode after the latter has been heated up by the discharge.

2. An electric gaseous discharge device comprising a sealed envelope containing a gaseous atmosphere consisting in part of vaporizable ma-- terial, at least three electrodes within said envelope, one of said electrodes being at one end of said envelope but at different distances from the first electrode, the electrode which is nearer said first mentioned electrode being an unactivated electrode of a refractory metal capable of thermionic emission when heated while the more remote of said electrodes is an activated thermionic cathode, a resistance connected between the two latter electrodes and traversed only by the current to the activated cathode which facilitates the transfer of a discharge from the activated starting electrode to the unactivated running electrode after the latter has been heated up by the discharge and a shield extending across the discharge path within said envelope close to the unactivated electrode on the side away from the activated starting cathode, said shield having a small central opening therethrough which constricts the discharge in the vicinity of the unactivated electrode and thereby increases the heating thereof by the discharge.

WILLEM ELENBAAS. AART VAN WYK. 

